The peak expiratory flow (PEF) has a linear decrease with age in patients with spinal muscular atrophy (SMA) and can be used as a progression marker for the disease, a new study in the Oprhanet Journal of Rare Diseases found.

The prospective cohort study led by Veldhoen et al included 80 patients aged between 4 and 66 years with a previous diagnosis of SMA types 1c to 3b. To assess the respiratory muscle strength, noninvasive spirometry values were obtained. A linear decline of 1% to 2% of PEF was reported.

SMA types 1c, 2a, and 2b had reached levels of PEF up to 80% below in early childhood vs type 3a that had reached it during teenage years. Maximal expiratory pressure (PEmax) was significantly low, with measures of less than 80 cmH2O in almost all patients, and a decrease could also be noted throughout the years.


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After measuring the maximal inspiratory pressure (PImax), the PEmax/PImax ratio was less than 1 in all SMA types and ages, suggesting that expiratory muscles are more severely weakened. Peak cough flow varied from normal to abnormal values according to the severity of SMA. Type 1c was well below 160 L/min, while type 3b did not register alterations, and the sniff nasal inspiratory pressure also appeared more drastically decreased in the most severe types.

Respiratory muscle weakness that presents progressively is common in patients with SMA and represents a significant cause of disability in these patients and leads to death. Although its pathophysiology is well known, a clear and objective measure has not yet been established.

“There are clear differences in respiratory muscle strength and its progressive decline between SMA types. In general, measurements of respiratory muscle strength are most affected in the more severe SMA types,” the authors concluded. “PEF declines in a rather linear pattern in all SMA types and is among the most suitable measures to be used for the longer-term follow up of patients and treatment efficacy assessments.”

Reference

Veldhoen E, Wijngaarde C, Hulzebos E et al. Natural history of respiratory muscle strength in spinal muscular atrophy: a prospective national cohort study. Orphanet J Rare Dis. Published online February 21, 2022. doi:10.1186/s13023-022-02227-7